The present invention relates to an electron gun assembly for a cathode-ray tube (CRT), and particularly to an electron gun assembly in which two gun modules are fabricated separately and then joined together to form an electron gun.
U.S. Pat. No. 4,400,644 issued to J. R. Hale on Aug. 23, 1983 describes a conventional electron gun such as that shown in FIGS. 1 and 2, fabricated by a single step beading operation in which metal support tabs or claws on each of the electrodes and other elements of the electron gun are embedded into a pair of oppositely disposed glass support beads. The support beads include at least two indexing cavities formed in the outer surface to properly locate the support beads on a beading apparatus such as that described in U.S. Pat. No. 4,341,545 issued to J. R. Hale on July 27, 1982.
A drawback of the conventional beading operation is that there is no chemical bond between the metal tabs or claws and the glass support beads. Anchoring of the various electrodes and other elements relies on entrapping glass between the claws to mechanically secure the electrodes and elements to the glass beads. Additionally, the method of embedding the metal tabs or claws within the support beads can induce misalignment of the electrodes. As described in U.S. Pat. No. 4,341,545, issued to Hale, the support beads are placed on beading blocks and the beads are heated until the glass softens sufficiently to permit the tabs or claws to be embedded therein. Then, the beading blocks are swung toward a mandrel holding the various electrodes and other elements of the electron gun. The molten support beads contact the metal tabs or claws which penetrate the support beads. Thermal and mechanical shock during the beading operation and the subsequent cooling may act to misalign or distort the various electrodes.
U.S. Pat. No. 4,298,818 issued to H. E. McCandless on Nov. 3, 1981 describes an improved electron gun in which a plurality of cathode assemblies, a control grid electrode (G1) and a screen grid electrode (G2) are all individually attached to a single ceramic member to form a beam-forming region (BFR) sub-assembly which is then attached, for example by welding, to a support bracket which was previously beaded, along with a main electron lens assembly, into a pair of glass support beads. The advantage of the patented structure is that the control grid and screen grid electrodes can be very accurately aligned and attached to the ceramic member, for example by brazing. A drawback of the patented structure is that the accuracy with which the support bracket and the main electron lens can be attached to the glass support beads is not substantially improved since a conventional beading apparatus is used to embed these elements into the support beads.
A further problem with a conventional electron gun structure is that electrical flashover (arcing) sometimes occurs from the lower end of the gun to a high voltage electrode (G4). The arcing occurs in the channels formed between the support beads and the neck of the tube envelope. U.S. Pat. No. 4,288,719 issued to K. G. Hernqvist on Sept. 8, 1981 discloses providing a conductive area on the surface of the support beads facing the neck in the vicinity of a focusing electrode (G3). The conductive area has the effect of neutralizing the longitudinal electric field in the channel, thereby reducing the longitudinal current in the channel, at least to the point where arcing is suppressed substantially. While the structural and arc suppressing improvements described in the aforementioned patents of Hale, McCandless, and Hernqvist have improved the manufacturability, alignment and performance of electron guns, further improvements are necessary in order to reduce the cost of the cathode-ray tube and improve its reliability and performance.